Boilers commonly use superheaters and reheaters to achieve higher output performance and thereby increase the efficiency of their operation. Typically, superheaters and reheaters comprise metallic piping which is often formed from ferritic and/or austenitic alloy steel and, particularly, stainless steels such as SS321 and SS347, among others.
The piping in the superheaters and reheaters of boilers operate in a high temperature environment for extended periods during which a two layer oxide develops on the inner bore of the tubes which is in contact with the steam. The layers of oxide together are, in some respects, technically desirable as they serve to protect the tube material from further corrosion by forming a protective layer.
FIG. 1A shows the oxide layers formed on the inner surface of a superheater or reheater tube 10. The oxide protective layer comprises two distinct layers, an inner oxide layer consisting of a fine grained Fe—Cr spinel oxide 12 adhering directly to tube material 11, and an outer oxide layer closest to the tube bore centre in contact with the steam inside the tube consisting of columnar-grained, porous magnetite 13 (being Fe3O4 often with some content of haematite Fe2O3).
As indicated in FIG. 1B, the two oxide layers have very different coefficients of thermal expansion and, in certain tubing compositions, coupled with large temperature fluctuations, such as when a boiler is cooled down, the magnetite layer 13 will delaminate from the Fe—Cr spinel layer 12, forming flakes of exfoliated magnetite. While the magnetite and spinel layers together are desirable for their corrosion protection characteristics, exfoliation of magnetite can be extremely problematic, particularly if it occurs in pendant type superheaters and reheaters, as the exfoliated magnetite will fall and collect in the bends at the bottom of the pendant resulting in blockage of the tube. FIG. 2 is a schematic representation of a typical high temperature boiler 20 with pendant superheater and reheaters. The collection areas for exfoliated magnetite are indicated by numeral 22.
Such blockages cannot be detected by ordinary means. Pipe blockage results in impeded steam flow causing overheating of the affected area of the superheater or reheater pipe, followed by creep rupture and failure of the tube. Pipe failures in superheaters and reheaters rapidly lead to secondary damage by steam erosion to adjacent tubes and consequently further failures follow quickly, until steam leakage within the boiler reaches a level where it cannot be tolerated and the boiler must be shut down to make repairs. The costs of lost production due to a single superheater or reheater tube failure can amount to millions of dollars per occurrence.
Prior art solutions to the problem of magnetite exfoliation typically tackle the problem by one of two approaches. The first is preventative, through the selection of special tube materials for use in the superheaters and reheaters at the time of original manufacture which tend not to form thick magnetite layers and accordingly help reduce exfoliation. The second approach is reactive to the existing problem by way of proposing methods to remove magnetite from tubes once it has already formed into an exfoliable layer or has already exfoliated as magnetite flakes.
One prior art preventative solution is the use of special stainless steel compositions having fine grain structures which exhibit a reduced propensity for the formation of exfoliated magnetite, such as stainless steel 347HFG, amongst others. While the manufacturers of this material have promoted the benefit of reduced magnetite exfoliation there has been at least one technical paper published documenting the occurrence of tube blockage after magnetite exfoliation from a superheater fabricated from this material.
Another prior art preventative solution is the use of a shot peening treatment for tubing during manufacture which modifies the grain structure of the tube material on the inner bore, as above, supposedly achieving a reduced propensity for the formation of exfoliated magnetite during operation.
Each of the above mentioned approaches employing a preventative solution generally results in at least a degree of reduction of magnetite exfoliation if implemented prior to fabrication of the boiler superheaters and reheaters. However, for the many existing superheaters and reheaters constructed from tube materials not preselected for the avoidance of magnetite exfoliation, none of the above techniques can be applied in situ, nor conveniently or economically retro-fitted.
Prior art solutions which are reactive in nature respond to the occurrence of magnetite exfoliation by the employment of methods to effect the deliberate physical removal of the exfoliated magnetite flakes. One such prior art solution is that disclosed in Australian Patent No 748326, in the name of C S Energy Limited, which describes a system for the removal of exfoliated magnetite during plant shutdown by fluidizing the exfoliated flakes in a carrier gas through the sudden controlled rapid depressurization of the boiler steam circuits, after charging to high pressure with a gas, thus conveying the flakes through the circuits to a desirable venting point for expulsion from the boiler.
Another reactive prior art solution is the cleaning of the superheater or reheater tubes using chemicals, including acids, which will dissolve the exfoliated magnetite flakes, as well as the delaminated but adherent magnetite layer. These can then be flushed from the boiler circuits thus removing the danger of exfoliated and loose magnetite. While chemical cleaning of this kind may be effective to remove magnetite it cannot be performed economically on a regular basis. A further consideration is that the plant's operational life is reduced by repeated removal and re-oxidation of tube material, leading to eventual thinning of the tube walls to the point where replacement is necessary. A further undesirable result of frequent chemical cleaning is the resultant large volumes of liquid waste containing dissolved metals, including chromium, which are difficult and/or expensive to dispose of in an environmentally satisfactory way.
Accordingly, it is desirable to provide an improved method for reducing the build up and/or subsequent exfoliation of magnetite in existing superheater and reheater tubes. This will result in a significantly reduced requirement for plant downtime for removal of exfoliated magnetite.